The present invention relates to clad piping and tubing, and more particularly to a composite billet for use in manufacturing clad piping and tubing and a method of manufacturing same.
One method of manufacturing seamless clad piping and tubing is to hot co-extrude a composite billet at high temperature in an extrusion press. A common technique for manufacture of other seamless pipes and tubes. The cylindrical extrusion billet is a composite of carbon or low alloy material on the outside and a corrosion resistant (xe2x80x9cCRAxe2x80x9d) alloy on the inside or vice versa. The range of sizes, wall thicknesses and alloy combinations available in the final product is restricted by the nature and production techniques of the composite billet that is used.
In one exemplary process for billet production as described in Osborn, U.S. Pat. No. 5,988,484, the disclosure of which is incorporated herein by reference, the starting CRA and carbon steel (xe2x80x9cCSxe2x80x9d) cylinders are machined to pre-calculated dimensions that allow for an accurate interference fit. When the CS outer cylinder is heated, it expands at the interface position creating a gap and clearance for it to slip over the CRA inner cylinder. As the assembly cools to room temperature the carbon steel contracts creating an interference fit with the CRA inner cylinder.
Another cladding process describing an outside diameter or OD clad pipe product is disclosed in Sponseller, U.S. Pat. No. 5,558,150, the description of which is incorporated herein by reference. The process is based on centrifugal casting both the clad material and support material, in sequence, to form a composite billet with the support material mechanically lining the CRA material, and without creating a bond between the two materials. As described more fully in Sponseller, the method seeks to inhibit metallurgical bonding and interdiffusion between the support and clad layers by strictly controlling the temperature and time interval between which the layers are consecutively poured.
Several drawbacks have been observed with these processes. For example, during heating of the composite billet in preparation for hot extrusion, the support carbon steel billet material and the CRA cladding material can grow or expand differentially (i.e., at different rates), with the interface between them opening up, as they are only interference fit or mechanically lined rather than metallurgically bonded to each other. This can cause the extrusion to fail, as the CS and CRA materials tend to extrude independent of each other. This is particularly true for composite billets fabricated from two materials with significantly different high temperature thermal expansion and mechanical properties. When the mechanical property differences at extrusion temperatures between the support and clad materials exceed certain limits, the failure rate of extrusions of composite billets increases dramatically. Thus, metallurgical bonding between the support and clad materials in the composite billet substantially increases the likelihood of a successful extrusion.
Accordingly, there is a need for an improved composite billet and method for manufacturing the same. Desirably, such a billet and method of manufacture overcome the drawbacks and failures of known methods and are used to produce high quality composite billets for making clad piping and tubing. More desirably, such a method can be used with a wide variety of base materials and alloys, without adversely affecting the properties and characteristics of either the base material or the clad alloy.
A method for producing a composite billet contemplates using simple and separate steps to produce the starting components, assemble the composite billet and then through the use of a further step of Hot Iso-static Pressing (HIP), create a High Temperature Metallurgical Bond (HTMB) of the billet interfaces prior to extrusion.
The outside support billet can be formed by any technique that can produce a hollow, preferably cylindrical, section. It can be formed from a hollowed or trepanned ingot, a forged, upset, extruded or ring rolled section from such ingot or from a centrifugal casting. Generally, the most cost-effective method of producing the required wall thickness and length of such a cylindrical section will be selected for use. It is not important that the section be forged, as further extrusion during clad piping manufacture will further consolidate the cast microstructure. This support section is finished, such as by machining, to the proper dimensions of the required support material for the assembly of the composite billet.
Similarly, the CRA cylinder that is fitted on to the inner surface of the support cylinder to produce the composite billet, can also be formed by a number of techniques. It can be formed from a hollowed or trepanned ingot or bar, an extruded section or from a centrifugal casting. Again, the most cost-effective method of producing the required wall thickness and length of this CRA cylindrical section will be utilized. Since this section is also further consolidated by extrusion, it is not important that the section be of wrought microstructure. This CRA section is finished, such as by machining, to fit with slight clearance inside the support carbon or low alloy cylinder.
A method of controlling the dimensions of extruded clad piping or tubing includes the steps of providing a support billet and a CRA billet of accurate dimensions, to provide a predetermined amount of base and clad material in forming a composite billet, with the clad material metallurgically bonded to the support billet. The amount of clad material is predetermined based upon the desired inside or outside diameter of the extruded piping or tubing. The composite billet is finished, such as by machining, to precise, predetermined inside and outside dimensions, and the composite billet is extruded.
Without being held to theory, it is believed that the metallurgical bond between the support billet and the CRA inhibits separation of the support billet and cladding material during subsequent hot extrusion of the billet into the clad tubing or piping product. In the absence of such a metallurgical bond, the clad material and the support material, which generally have different high temperature tensile properties and coefficients of expansion, can expand to different degrees. This causes the interface between the materials to open up and the extrusion to fail. In some cases, the clad and support materials can extrude independent of each other resulting in extrusion failure. Such failures become especially pronounced when there are large differences in the high temperature tensile properties of the two materials that are being coextruded, as for example in the case of a carbon steel support and a CRA that contains high nickel and other alloying elements that provides high temperature strength.
By metallurgically bonding the clad material to the support material in forming the composite billet, the present process overcomes many of the difficulties of known composite billet forming processes. The invention also avoids mixing and pickup of alloying elements into the support material from the clad and vice versa and further avoids precipitation of second phases and defects at the interface of the support and clad materials. The invention also allows for a wide range of clad and support materials to be used and results in an economical method of forming clad piping and tubing.